Light-emitting diode (LED) based illumination devices are increasingly used for a wide variety of lighting applications. LEDs offer advantages over traditional light sources, such as incandescent and fluorescent lamps, including long lifetime, high lumen efficacy, low operating voltage and fast modulation of lumen output.
Efficient high-power LEDs are often based on blue light emitting materials. To produce an LED based illumination device having a desired color (e.g., white) output, a suitable wavelength converting material, commonly known as a phosphor, may be used which converts part of the light emitted by the LED into light of longer wavelengths so as to produce a combination of light having desired spectral characteristics. The wavelength converting material may be applied directly on the LED die, or it may be arranged at a certain distance from the phosphor (so-called remote configuration). For example, the phosphor may be applied on the inside of a sealing structure encapsulating the device.
Many inorganic materials have been used as phosphor materials for converting blue light emitted by the LED into light of longer wavelengths. However, inorganic phosphors suffer from the disadvantages that they are relatively expensive. Furthermore, inorganic phosphors are provided in the form of particles, thus always reflecting a part of the incoming light, which leads to loss of efficiency in a device. Furthermore, inorganic phosphors have limited quantum efficiency and a relatively broad emission spectrum, in particular for the red emitting inorganic phosphors, which leads to additional efficiency losses. Currently, organic phosphor materials are being considered for replacing inorganic phosphor in LEDs where conversion of blue light to green to red light is desirable, for example for achieving white light output. Organic phosphors have the advantage that their luminescence spectrum can be easily adjusted with respect to position and band width. Organic phosphor materials also often have a high degree of transparency, which is advantageous since the efficiency of the lighting system is improved compared to systems using more light-absorbing and/or reflecting phosphor materials. Furthermore, organic phosphors are much less costly than inorganic phosphors. However, since organic phosphors are sensitive to the heat generated during electroluminescence activity of the LED, organic phosphors are primarily used in remote configuration devices.
The main drawback hampering the application of organic phosphor materials in remote phosphor LED based lighting systems is their photo-chemical stability, which is poor. Organic phosphors have been observed to degrade quickly when illuminated with blue light in the presence of oxygen.
U.S.2007/0273274 (Horiuchi et al.) discloses a translucent laminate sheet comprising a light-emitting device and comprising an organic phosphor arranged in an airproofed space. The space is filled with the organic phosphor in a state where the concentration of oxygen is kept at 100 ppm and preferably at 20 ppm or less in a vacuum or ambient atmosphere of inert gas, to avoid deterioration of the phosphor. However, performing this operation under such low concentrations of oxygen is difficult and costly.
Hence, there remains a need in the art for improved light-emitting devices employing organic phosphor materials.